Electrical current sensing systems may measure current (AC or DC) through a current carrying wire by measuring the magnetic field associated with the current carrying wire. For example, current sensing systems may determine the direction and magnitude of the current through the wire based on the magnitude and direction of the magnetic field associated with the wire. In some current sensing systems, the current carrying wire may be placed adjacent to a ferromagnetic core that defines a gap region in which the magnetic field generated around the wire is concentrated. The concentration of the magnetic field in such a gap region may provide a location at which the magnetic field generated by the current carrying wire may be reliably measured.
A variety of different current sensing systems may use a ferromagnetic core that defines a gap region, as described above. Current sensing systems using such an arrangement of a current carrying wire and ferromagnetic core may be described as open loop sensing systems or as closed loop sensing systems. In an open loop sensing system, the current through the current carrying wire may be determined by measuring the magnetic field in the gap region. In such an open loop system, a magnetic field sensor is located in the gap region and the wire is located adjacent to the core such that the current through the wire generates a magnetic field that is concentrated in the gap region. A circuit of the open loop sensing system may measure signals generated by the magnetic field sensor and determine the current in the current carrying wire based on the signals generated by the magnetic field sensor.
A closed loop sensing system may be used instead of the open loop sensing system described above. A closed loop sensing system may include the wire, core, and magnetic field sensor in the gap region of the core, as described above with respect to the open loop sensing system. Additionally, the closed loop sensing system may also include a secondary coil that may be wrapped around the core. This secondary coil may receive current from a control circuit and generate a magnetic field in the gap region in addition to the magnetic field generated by current through the current carrying wire. The control circuit of the closed loop system may receive signals from the magnetic field sensor in the gap region and generate a current in the secondary coil that attempts to zero out the magnetic field in the gap region. The amount of current required to zero out the magnetic field in the gap region may indicate the magnitude and direction of the magnetic field in the gap region. The magnitude and direction of current through the current carrying wire may be determined based on this current generated by the control circuit, e.g., determined based on the number of turns in the secondary winding relative to the number of turns in the current carrying wire.